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Re: [Phys-L] Figuring Physics solution Jan 2018

Since we a rephrasing Hewitt's question so it fits our answers, let's try another example.

I have a beaker containing 1 liter of water at 40 C. I pour out 50 ml. Has the temperature of the water in the beaker changed?

Hewitt's question is no more complicated nor deep than that. His proposed response choices are more than adequate.


From: Phys-l <> on behalf of John Denker via Phys-l <>
Sent: Thursday, January 25, 2018 5:31:04 PM
To: Forum for Physics Educators
Cc: John Denker
Subject: Re: [Phys-L] Figuring Physics solution Jan 2018

On 01/25/2018 01:16 PM, Robert Cohen gave a summary.

I agree with what he said, but I might say it in slightly
different terms, with slightly different emphasis:

1) It is super-important to distinguish between energy
and temperature. Obviously energy is extensive, but
there is a deeper point to be made about energy
/density/, which is intensive, as is temperature.
A pot of boiling water is the poster child for a
process that has a changing energy per unit mass
at constant temperature.

Let's postpone the discussion of what the author should
have said, and focus on what he actually said. He used
the language of temperature, namely cooling and warming.
The tail of the question is:

[...] evaporation would be a
A. cooling process.
B. warming process.
C. Neither of these.

You don't even need to read the first part of the question
to realize that A might be true, but not usually, and B
might be true, but not usually. Therefore C is by far
the best answer, because it is much broader and therefore
more likely to contain the right answer, whatever that
may be, and is not itself excluded in any obvious way.
In particular, it covers the poster-child case, i.e. a
pot of boiling water.

Under this interpretation, the first small part of the
given answer, i.e. the qualitative part, is correct.
However, the main part of the answer is wrong. The
point he is trying to make is wrong, and the path he
takes to get there is fallacious.

2) Working backwards from the answer, we surmise that
the author may have wanted to talk about energy or
energy density rather than temperature. So to be
charitable, let's consider the emended question:

[...] the process of evaporation would be:
A'. absorbs energy.
B'. gives off energy.
C'. neither of the above.

Still working backwards, we see that C' is the desired

The answer cannot possibly be correct, because there
are no such liquids. If it had zero latent heat, it
it would have no binding energy, so it would be a gas,
not a liquid.

This is a fallacy so old that there is a Latin term
for it: Ex contradictione quodlibet.

In other words, if you imagine something that does
not exist, you are free to ascribe to it whatever
properties you like. You must not, however, imagine
that it has anything to do with the real world. In
particular, it tells you precisely nothing about the
behavior of real liquids.

This time you don't need to read /any/ of the
question to know that the answer is wrong. The
evident intent of the question was to create a
situation that does not exist, and the details
of how that was done aren't really important.

3) There is a prevalent and pernicious misconception
that "thermal energy" (whatever that means) is a
form of microscopic kinetic energy, to the exclusion
of potential energy.

The author didn't say anything about PE, and a lawyer
might argue that if you didn't say anything you didn't
say anything wrong ... but students aren't lawyers.
If you talk about KE and nothing else, students will
assume that what matters is KE and nothing else.

In fact, during evaporation, you do not have high-
speed particles departing from deep in the bulk.
Instead you have a multi-step process whereby a
particle makes its way into the surface layer,
gradually (but not monotonically) reducing the
number of bonds it has, i.e. reducing its PE,
until it eventually breaks free. In contrast,
the alleged model in terms of KE only:
a) paints a wildly wrong picture of liquids in
particular, and
b) tends to spread and exacerbate misconceptions
about thermodynamics in general.

Again you don't have to read the question to know
the picture he is trying to paint is mostly wrong.

*) In summary, we have sloppy ideas and fallacious
logic leading to a wrong and deceptive conclusion.

There's a lot more one could say (e.g. about the
question itself) but I see no immediate need to
flog other parts of the dead horse.
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